Food packaging has changed radically over the last 50 years. These changes have allowed manufacturers to market food products to consumers in new forms. Entire new categories, such as frozen prepared foods, refrigerated prepared foods and packaged shelf stable prepared foods have been created. These new product types are sold in very large numbers as consumer items in a national marketplace. They require packages and sealing techniques usable in a mass production environment which are inexpensive and reliable.
The products described are produced at large plants for distribution over large areas. Of necessity, these plants prepare and package very large numbers of products in an automated manner with the automated equipment running continuously at a very high production rate. With respect to packaging, such a production system requires that containers with a prepared food product be handled by an automatic closure machine which applies a closure, seals the closure to the container and delivers the closed container to a boxing station or the like in a repetitive, high rate manner. Reliability is very important in such an operation.
Sealed plastic containers are one form of packaging which has grown very rapidly over the last several years. Such containers are used for a wide variety of foods including shelf stable prepared foods. Often, it is necessary to assure a complete seal between the container and its closure which will remain tight during shipping and marketing, but be easily opened by the consumer. Such sealing between a closure and a container is not easily achieved in the high production rate automated packing lines prevalent in the modern food industry.
Another aspect of the modern food industry is the rapidity with which food products are changed. Recipes are updated regularly, product lines are changed to cater to consumer tastes and package sizes and shapes are changed. A food company must maintain versatility of manufacturing technique in order to stay competitive.
The achievement of a complete seal between a package and closure reliably in the above-described setting has been a problem. One method of obtaining a seal uses induction heating of the area to be sealed. A food filled package of tray shape is positioned in the sealing appliance. A closure or lid is placed on top of the package in its final position. Either the closure or the container is provided with a band of conductive material defining the area in which a seal is to be created. Often, this conductive material is a band of aluminum around the periphery of the closure. A band of hot melt adhesive may be added over the aluminum. An induction heating fixture is brought into close proximity or contact with the assembled container and closure and energized. An electric current is induced in the conductive aluminum band, heating the closure and/or container where a seal is desired to a point where these plastic elements or the adhesive liquify sightly. The container and closure are thereby cohesively bonded, adhesively bonded or welded together. One such induction heating food package sealing apparatus is described in U.S. Pat. No. 4,707,213 to Mohr et al.
While the above-described process has achieved success in many respects, problems still exist. Variations in positioning of the container and/or closure at the induction sealing station can cause variations in the seal. Slight defects in the application of the aluminum conductive material can cause local variations in heating and therefore variations in the seal. Other factors such as misalignment between the inductor and the aluminum band and the like also cause local variations in the sealing of a closure to a package. This can cause some inconsistencies in sealing of packages.
These and other problems are overcome by the present invention wherein a method and apparatus for improved control of induction heating and package sealing is provided.